1. Field of the Invention
This invention relates to a process for preparing alkylthio- and arylthio-substituted carboxylic acids and their corresponding thioesters, comprising reacting a mercaptan with a lactone in the presence of an acidic catalyst. In the preferred embodiments of the present invention, the process provides continuous production and batch production of thio-substituted carboxylic acids and the corresponding thio-substituted thioester.
2. Description of the Prior Art
Alkylthio-substituted carboxylic acids and arylthio-substituted carboxylic acids (hereinafter sometime jointly referred to by the phrase "alkyl- and arylthio-substituted carboxylic acids") are important synthetic intermediates for a wide range of product applications. For example, methionine (2-amino-4-methylthiobutyric acid) and its hydroxy analog, 2-hydroxy-4-methylthiobutyric acid, are widely used as animal-feed supplements. In addition, sulfone peroxycarboxylic acids have been reported as a novel class of stable, solid, bleach materials for detergent applications, as disclosed in U.S. Pat. No. 4,758,369.
The reaction of lactones, for example, .gamma.-butyrolactone, with various alkyl metal mercaptans and aryl metal mercaptans (i.e., mercaptide ions), typically conducted in the presence of a co-solvent, for example dimethylformamide (DMF), dimethylsulfoxide (DMSO) and hexamethylphosphoramide (HMPA), to facilitate solubilizing the mercaptide ion, is well known in the literature. Typical mercaptides comprise sodium and potassium metal. The cleavage of .alpha.-aminobutyrolactone with sodium methyl mercaptide, followed by acidification of the resulting sodium carboxylate to produce methionine or its derivatives, is described in several references, for example, Chem. Ber., 83, pp. 265-268 (1950); U.S. Pat. No. 4,311,849 and British Patent Application 651,165 (Chem. Abst. 51:2853c). In addition, other ring opening reactions of various lactones with mercaptide ions or metal salts of selenium compounds (i.e., selenide ions) have been reported, for example, U.S. Pat. Nos. 2,842,587; 2,842,590 and 2,872,458; Tet. Letters, p. 4369 (1977); Tet. Letters, p. 4361 (1977); Chem. Ber. 94, p. 2060 (1961); Tet Letters, p. 3859 1977; J. Organometal. Chem., 25, p. 33 (1970).
Preparation of the mercaptide ion typically comprises reacting the mercaptan with a metal hydride, for example, sodium hydride (NaH), potassium hydride (KH) or lithium hydride (LiH). Alternatively, the metal mercaptan may be prepared by reacting the mercaptan with a metal, for example, potassium metal (K) or lithium metal (Li). The use of metal hydrides, for example, NaH, is well known to those skilled in the art of organic synthetic chemistry. However, the use of metal hydrides often proves to be problematic due to their flammability and moisture sensitivity. Such drawbacks often require maintaining the metal hydride under an inert atmosphere during handling, as well as maintaining an inert atmosphere over the reaction medium. Similarly, metals, such as K, are difficult to handle since they are flammable and moisture sensitive. Therefore, the present invention substantially eliminates the need to use mercaptide ions to prepare thio-substituted carboxylic acids and thio-substituted thioesters from lactones.
Several papers report the use of Lewis acid complexing agents, for example, aluminum chloride, to facilitate carbon-oxygen bond cleavage in esters and lactones. See, for example, J. Org. Chem., 45, p. 4275 (1980); J. Org. Chem., 46, p. 5163 (1981) and Tet. Letters, 52, p. 5211 (1978). In all of these cases, the Lewis acid is used in stoichiometric amounts. While the use of Lewis acid complexing agents, such as aluminum chloride, is not as problematic as the use of metal hydrides and metals when preparing metal mercaptans, they are generally moisture sensitive and corrosive. The present invention substantially eliminates the need for such Lewis acid complexing agents when reacting mercaptans with lactones.
Moreover, although free mercaptans are known to ring-open highly strained lactones, for example, propiolactone (J. Org. Chem., 39, p. 2648 (1974)), there appear to be no examples in the literature of similar reactions of higher lactones, for example, butyrolactone and valerolactone. The present invention provides a method for ring-opening such higher lactones with free mercaptans.